You might try checking on your server for guilds with several hundred members. My sense is that these large guilds usually have a mix of people really into raiding and others who are more casual. You can sign up for raids on nights you are available or there are often enough guild members on to get a 5 or 10 person going.

Before Cataclysm Arcane was the raiding spec that did the most dps. So your guild mate probably kept arcane based on that experience. Now it looks like more dps is done in raids by mages with fire spec.